Add custom pypi URL to extension point

[cdancy]

Hey ... is this project still being maintained? If so would it be possible to expose a property on the jython extension to set a custom pypi URL? We'd like to use our Artifactory instance as a pypi proxy as we've found pypi to sometimes be flaky. I can send in a PR to knock this out if you'd take it in and kick a new release.

[rzabini]

Hello,
sure, if you send a PR I will integrate it in a new release, thank you.